Transconductance

Recall the discussion of dynamic current amplification from the last chapter. This is a measure of how well a bipolar transistor amplifies a signal. The JFET analog of this is called dynamic mutual conductance or transconductance.
Refer again to Fig. 1. Suppose that E_G is a certain value, with a corresponding I_D resulting. If the gate voltage changes by a small amount dE_G then the drain current will also change by a certain increment dI_D. The transconductance is the ratio dI_D/dE_G. Geometrically, this translates to the slope of a line tangent to the curve of Fig. 1.
The value of dI_D/dE_G is obviously not the same everywhere along the curve. When the JFET is biased beyond pinchoff, in the region marked Y in the figure, the slope of the curve is zero. There is no drain current, even if the gate voltage changes. Only when the channel is conducting will there be a change in I_D when there is a change in E_G. The region where the transconductance, dI_D/dE_G, is the greatest is the region marked X, where the slope of the curve is steepest. This is where the most gain can be obtained from the JFET.

Fig. 1: Relative drain current as a function of gate voltage for a hypothetical N-channel JFET